Bruise in leg 2 inches wide n 2 inches long has clear center. no injury. what might cause this?

Bruise: Most likely this was either from some trivial trauma or a spontaneous leakage of blood from a skin capillary. This should slowly resolve. If you have frequent unexplained bruising,bleeding from gums or nose, get checked for a hematology problem. Taking aspirin or medications like corticosteroid, Advil and Aleve can make you bruise easily.